Fix pytest script-based test invocation

test/test_maxima.py

@@ -143,5 +143,5 @@ if __name__ == "__main__":
     if len(sys.argv) > 1:
         exec(sys.argv[1])
     else:
-        from py.test.cmdline import main
+        from pytest import main
         main([__file__])

test/test_pymbolic.py

@@ -492,7 +492,7 @@ if __name__ == "__main__":
     if len(sys.argv) > 1:
         exec(sys.argv[1])
     else:
-        from py.test.cmdline import main
+        from pytest import main
         main([__file__])
 
 # vim: fdm=marker

test/test_sympy.py

@@ -139,5 +139,5 @@ if __name__ == "__main__":
     if len(sys.argv) > 1:
         exec(sys.argv[1])
     else:
-        from py.test.cmdline import main
+        from pytest import main
         main([__file__])